Tuesday, April 2, 2019

Random Wire Antenna Project Getting Started

I'm just getting going here putting up a permanent wire antenna. I really do need something better than the BuddiStick I've been using for several reasons. These antennas are really meant for temporary deployment, usually during portable operation. While they're built very well (the best I've seen!), they really don't stand up to 24/7/365 outdoor use here in Northern Colorado. Yes, I know a lot of people use them that way, and they've come up with ingenious ways to weather proof them, I just don't want mine to get all trashed when it doesn't have to. So, I'm tossin' up a big chunk of wire, and gonna load that up!

I can't say enough good things about the BuddiPole/BuddiStick antenna and the people that make them. These things are the Legos of the antenna world. You can almost configure them too many ways, but it's great fun to experiment with. They take a while to understand, and even longer to grok, but once you do you can build some very efficient antennas in a quite small space. Their customer service is exemplary, and the support for these things is phenomenal. Look into them if you need a small, light, portable antenna system and like to tinker with antennas.

While my BiddiStick 1/4-wave vertical is very effective on 20 Meters (14MHz band), it's physically too short for lower frequencies, and presents a non 50 Ohm impedance at higher frequencies. The "antenna tuner" add-on to my little Elecraft K2 is a wide-range "tuner", and can easily handle a mismatch of up to 10:1 VSWR. It's tuned anything I've ever connected to it, allowing the radio to put out full power into a 50 Ohm load. The "tuner" in the FT-1000D is designed to match up to a 3:1 mismatch, which basically means that it only works with an antenna that's already a fairly good match to 50 Ohms. Lots of people more properly refer to built-in "tuners" like this as "Line Flatteners", meaning the smooth out the bumps and ripples in an antenna system that's already in pretty good shape. In these cases, the "tuner" is at the radio, and a length of feedline (usually coaxial cable) runs to the antenna.

Why am I putting "tuner" in quotes? Because regardless of what people call these things ("Coupler" is the correct term), they cannot "tune" an antenna. The only way you can tune an antenna is by changing it physically; Length, height above ground, orienation, these are what you do to tune an antenna.. What all these devices do is to provide an impedance match between the feedline and the transmitter, enabling the transmitter to "see" a matched load and transfer maximum power. How well the antenna accepts that power and radiates it is anybody's guess. The important thing is that the transmitter is connected to a matched load and will happily hum along at maximum power. This also results in the feedline having anywhere from 1:1 VSWR to 20:1 (or higher) VSWR. Depending on the length and inherent loss of the cable used, very high (>10:1) VSWR can cause additional line loss. Coaxial lines with high VSWR also have a reputation for radiating (the cable becomes part of the antenna system), causing whatever pattern you thought you antenna had getting very distoted, and the resulting in RF in the shack, which is a whole 'nother article.

When the impedance matching device (aka "tuner") is located at the feed point of the antenna, different things happen.

#1, the load impedance presented to the transmitter is very nearly 50 Ohms, allowing the transmitter to deliver full power,

#2, the VSWR on the coaxial cable feedline is very close to 1:1, keeping voltage stress down,

#3, the chances of current flowing on the outside of the coax, making it radiate, are greatly diminished,

and #4, It'll tune damn near anything!

So, wire antenna supported as high as I can comfortably get it, fed by my trusty SGC-230 autocoupler.

And after much discussion with my buddy the Wire Antenna Guru, we've come up with some numbers that should work. I'll be running an 88' "Random Wire" antenna, with the ends about 15' off the ground, and the center at about 25'. This will be an "End Fed" antenna (otherwise it would be an off-length dipole!), and my SGC-230 autocoupler will be mounted on the fence at one end, with the wire connected to it. I'll run the ground radials along the fence about 4'~5' above the ground, giving me "Elevated Radials", which mitigates a lot of the ground loss. This is the loss that I saw when I had this coupler connected to the 33' vertical with only 4 radials laying on the ground. It would find a match, but it just didn't radiate very well compared to back in Long Beach where the radials were 8' above the ground.

The approximate center of the antenna will be about 25' high, using the 15' stump of the cottonwood we had cut down as part of the center support. I'm going to bore an 1-1/4" hole in the top of the trunk, and drive two sections of mast into the hole. The wire will come from the feed end, up to the mast, through a pulley, and then on to the far end, where the support rope will go through another pulley, and down to either a weight or a spring to keep some tension on it.

So I made up a list of stuff, and took a trip to Home Depot today. The Brown Truck of Happiness delivered my order from DX Engineering this afternoon, and I now have (I hope) all the bits and pieces to get this put together and in the air.

So stay tuned, pix will follow!

5 comments:

  1. DrJim, I have found a good material for shorter masts, heavy duty chainlink toprail. It generally comes in 22 or 24 foot lengths. That way you could get your center up above 30 feet as heigth always seems to help. You have to go to a company that specializes in fence materials; it appears that ACI Supply is such a one in your area.

    I am using two pieces, one 22' with 10' bolted on it to get a 2 meter beam up at the top and supporting a 40 meter dipole at 27'. We have had some high winds and it is standing.

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    1. I've heard of people using that for masts.

      I already have a pile of mast I brought with me when I moved. Using it up for this project gets it off the shelf and out of the inventory.

      And I'm working on getting a 4-1/2' Glen Martin tower with an M2 2M9 9-element 2 Meter beam up on the peak of the roof.

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  2. IMO, too many hams look down their noses at antenna couplers, afraid to lose the couple of tenths of a dB insertion loss, overlooking everything else. In avionics HF systems, couplers are looked at as part of the system and designed in. If it's a permanent installation, yeah they probably have a budget to do a big installation with full sized antennas, but that's not the case for aircraft.

    Your analysis is exactly right, of course. The feed point is the place to put it. It minimizes the coax losses, which is the real problem with a higher SWR.

    FWIW, and for the readers who don't realize this, the 3:1 limit of VSWR isn't a hard limit. I've had several Icom rigs with those 3:1 line flatteners in them that all went far beyond that in many places - like 12 or 13:1.

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    1. Yep. Built-in tuners can be very wide range. The one in the Yaesu isn't. I have my LP-100A Vector RF Wattmeter at the output of the rig, and when I hit the "Start" button on the rig's tuner, I can see it attempt to tune with about 10 Watts, and then kick off if the VSWR is greater than 3:1.

      It would have been an interesting experiment to put it at the output of the Elecraft tuner to see what the LINE VSWR on all those frequencies where the antenna wasn't resonant.

      I can see one definite difference. With the K2 running around 75 Watts into a highly mismatched line, our 'touch controlled' lamp in the front upstairs bedroom would come on when I'd transmit on a band other than 20 Meters.

      Feedline radiation? Very probable.....

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Keep it civil, please....